Cardiac output is a calculation of the volume of blood being pumped by the heart, for example a ventricle, per minute. Cardiac output is equivalent to the heart rate multiplied by the stroke volume. Understanding of a subject's cardiac output is important in care of acutely injured or ill subjects, as well as individuals with chronic cardiac pathology. Until recently the standard of cardiac output measurement has been pulmonary artery catheterization. See, e.g., U.S. Pat. Nos. 3,915,155; 3,726,269 and 3,651,318.
Bioelectrical impedance analysis (“BIA”) has been developed to measure physiological and pathological properties, including cardiac output. In performing BIA, a low level electrical alternating current is introduced into a tissue being monitored electrically by multiple electrodes, such that the voltage difference between multiple locations on the tissue is determined. From this determination, the electrical impedance (electrical resistance plus reactance) of the stimulated tissue is calculated. Previously, both external (U.S. Pat. No. 4,870,578) and internal (U.S. Pat. Nos. 4,852,580 and 4,836,214) electrodes have been employed to measure electrical resistance representing blood flow in the aorta. While these internal electrodes were mounted on esophageal catheters, it was later determined that endotracheal tubes could be adapted by the addition of electrodes on the inflatable cuff, which was perceived to be a more accurate measurement of cardiac output. See U.S. Pat. Nos. 6,095,987 and 6,292,689.
The process of inserting an endotracheal tube is called intubation, and is performed\\\ when the inflatable cuff is deflated. The presence of electrodes on the inflatable cuff and electrode leads on the external surface of the endotracheal tube results in a more complex and riskier intubation process. Further, the electrodes are attached to the inflatable cuff when inflated, resulting in irregularities (e.g., sharp edges of the electrode, broken electrode leads) when the cuff is deflated prior to insertion.
In view of the foregoing, it would be desirable to provide an apparatus and methods for safely, accurately, efficiently and continuously determining cardiac output by measuring electrical impedance.